GSM Onboard Aircraft

For many years, airlines refused the use of cellular telephone technology in-flight. There was fear that, unable to make reliable contact with ground-based base stations, mobiles would transmit with maximum RF power and the ensuing high RF fields could potentially cause interference with both aircraft communications (which use a band harmonically related to the original GSMTM frequencies) and to aircraft flight control systems.

In addition, successful calls via the terrestrial GSM network would detract from the Terrestrial Flight Telecommunication System (TFTS) service that was being deployed commercially by many airlines. However, commercial demand for TFTS failed to sustain initial expectations, probably due to the perceived high cost of the service and the lack of the personalized features that users now enjoy in their personal mobile phones, and that service has now ceased.

More recently, Boeing has been offering its 'Connexion' service through selected airlines. Connexion provided Internet access to travellers. The system used a satellite link to connect to the ground and a wireless local area network (WLAN) onboard for the access the system. Again, due to the lack of market demand Boeing announced that the service would be discontinued at the end of 2006.

Technology issues

Early research showed that reasonably reliable mobile-network connections could be established for aircraft flying below about 3000 metres (i.e. during take-off and landing phases), and experience has shown that some connections are possible even at cruising altitude (around 10 000 metres). However, the high speed of the aircraft causes frequent handover from cell to cell, and in extreme cases could even cause degradation of 'legitimate' terrestrial services due to the large amount of control signalling required in managing these handovers.

It might be thought that the metal fuselage of an aeroplane would effectively attenuate radio signals at GSM frequencies. However, calculation - and indeed experiment - show that this is often not the case, and in fact the arrangement of windows and other apertures in the aluminium body can even act as an effective slotted antenna, causing surprisingly high field strengths in certain directions.

In 2005 a novel approach was suggested whereby a low-power base station would be carried on board the aircraft itself, and an associated unit would emit radio noise in the GSM band, raising the noise floor above the signal level originated by ground base stations. Thus mobiles activated at cruising altitude would not see any terrestrial network signal, but only the aircraft-originated cell. Power levels would only need to be low (since mobile and base station would be in very close proximity), and thus interference with aircraft systems would be minimized.

Liaison between the aircraft-borne base station and terrestrial network(s) would be via satellite, as with TFTS.

Regulatory considerations

Two recent developments in relevant European regulatory groups have increased the likelihood that the GSM onboard service can be successfully launched: the decision of TCAM (European Commission Telecommunication Conformity Assessment and Market surveillance committee) that the complete system falls under the R&TTE (Radio & Telecommunications Terminal Equipment) Directive, and the development of the spectrum regulatory framework in the Regulatory Affairs Working Group (WGRA) of the Electronic Communications Committee (ECC) of CEPT (the European Conference of Postal and Telecommunication Administrations). There has also been considerable consensus around a regulatory framework in the Asia Pacific region.

Regulatory developments within Europe commenced in CEPT WGRA in July 2004 when the draft framework was proposed. Consequently, the spectrum engineering group (CEPT / ECC / WGSE) was requested to investigate the compatibility of such a service with existing systems.

CEPT WGSE Project Team SE7 commenced the compatibility study in January 2005. The result of the work is an ECC Report, published in September 2006. This Report specifically addresses the impact of the GSMOB system on terrestrial GSM, UMTSTM (WCDMA (UTRA FDD)), FLASH-OFDM and CDMA2000 technologies.

On the basis of these regulatory developments mentioned and the SE7 report, CEPT WG RA produced and submitted for public consultation a draft ECC Decision on GSM onboard aircraft. The Decision, which was published in December 2006, will allow operation of a GSM onboard (GSMOB) system, compliant with the requirements laid out in its Annex, according to the licensing conditions for the use of spectrum in the country of registration of the aircraft. It also requires that the equipment onboard complies with the R&TTE Directive.

Service description

GSMOB mobile services will allow airline passengers to use their own mobile terminals during certain stages of flight. Passengers will be able to make and receive calls, send and receive SMS text messages and use GPRS functionality. The system provides mobile visited network access, meaning that the onboard network is run by a licensed operator with roaming agreements with the passengers' home operators and that the call will be billed to the user like any roaming call. For reasons that are mainly technical, the frequencies used for onboard communications are in the GSM1800 band.

System description

The terminals will connect to an onboard pico Base Station using the standard GSM radio interface. The onboard cell is connected to the terrestrial networks through a satellite link.

GSM onboard will be deployed in aircraft flying over terrestrial networks, very often over more than one country. The system must ensure that onboard terminals do not attempt to communicate or interfere with terrestrial networks, and the system itself must not interfere or attempt to communicate with ground terminals.

The system is operational during the top of ascent, cruise and commencement of descent phases of the flight, only when the aircraft is more than 3000m above ground level. During the phases of the flight that the service is not active, such as during take-off and landing, passengers must switch off their mobile terminals.

Viability of GSMOB

It is now accepted that it is technically feasible to offer a GSM service on-board civilian aircraft that is safe in terms of avoidance of interaction with aircraft systems, immune from interference with terrestrial mobile communications, compatible with current regulation (at least that of Western Europe), and capable of offering a service that responds to perceived customer demand.

Naturally, there are issues related to the offering of GSMOB that extend beyond the purely technical and regulatory. There are social issues, such as how a system can be managed to prevent annoyance to other passengers. Cabin crew are likely to have to acquire new skills to deal with such circumstances, as well as for managing the use of the system in the various phases of the flight. With the support of the airlines and the GSMOB operators, passengers may need to be taught a new 'social etiquette' for the use of mobile devices in-flight.

The aviation industry will need to address issues of aircraft and system type approval, taking account of the wide variety of aircraft configurations to be accommodated. Finally, of course, GSMOB will only be deemed a success if it meets passengers' demands and satisfies the commercial expectations of its promoters.

ETSI White Paper

A more complete discussion about GSMOB can be found in a free ETSI White Paper.